A common reactor configuration has a reactor shell having an internal cavity that is lined with porous scallops. A porous, vertical, hollow centerpipe fits within the internal cavity, and below a cover means, to form an annulus between the scallops and the centerpipe. That annulus is filled with catalyst. Reactants pass into the reactor through an inlet means within the reactor shell, passes around the cover means, through the scallops, into the annulus, where the reactants are reacted with the catalyst. The reacted products pass through the porous centerpipe, out an outlet means within the bottom of the reactor shell. The bottom of the centerpipe is designed to rest on a ledge within the outlet means.
The centerpipe fits into the outlet. For reasons not fully understood, but having to do at least in part to thermal cycling and the movement of catalyst past the centerpipe, the centerpipe tends to move upward, in an action termed "ratcheting," out of the outlet during use. In fact, the force moving the centerpipe upward is strong enough that, were the centerpipe attached to the reactor floor, using bolts or welds, the force would rip the centerpipe out of the outlet. As the centerpipe moves upward, a space between the pipe and the outlet develops, through which catalyst will flow out of the reactor.
This problem has been recognized for many years, and it continues to be important. Many solutions have been offered, none of them completely satisfactory. One proposed solution was to place a covered cylindrical dam within the annulus so that catalyst would not be lost when the centerpipe ratcheted up. This dam is filled with varying sizes of inert balls. Unfortunately, that solution does not completely prevent catalyst from escaping through the outlet. Some of the catalyst makes its way through the inert balls and out of the reactor. Also, if the centerpipe ratchets up, the inert balls might escape through the outlet.